Preparation Of Standard Antisera Against Different Serotypes Of Streptococcus Suis And The Identification Of Virulent Streptococcus Suis Type 9

Swine streptococcosis is an important disease of the second kind of loemia in China,and causes great economic losses in swine industry.More importantly,Swine streptococcosis also is an important zoonotic disease.In the middle of the last century,Holland and the United Kingdom have identified Streptococcu suis(S.suis)as the pathogen of this disease.Subsequently,this bacterial pathogen has been spread to many countries in other continents,including China,Vietnam,Japan,Thailand,Canada and the United States.At present,S.suis has recognized as one of the most important disease for the swine industry worldwide.Once the swine infected with S.suis,the mortality rate may reach to twenty percent if without effective treatment.The most important clinical feature of this disease is meningitis,even causes acute death.S.suis is a kind of gram positive cocci,in accordance with the capsular polysaccharide composition are different,and could be divided into 33 serotypes according to their different constitute in capsular polysaccharide.In China,the prevalent pathogenic serotypes causing animal disease were serotype 1,1/2,2,7 and 9.Especially,serotype 1 and 2 have been reported to infect human,and even cause death.Therein,S.suis serotype 2 has the highest rate in clinical isolation,and shows the strongest pathogenicity.In recent years,the prevalence of S.suis serotype 9 is increased gradually,and causes more and more attention.The diagnosis of pathogenic S.suis is mainly based on the bacteria isolation,and then identified by morphological,cultural and biochemical characteristics.Since the complexity of S.suis serotype,it is difficult to accurately identify the serotypes using the above methods,thus the serological method is an important technology for the specific identification.In this study,the bacterial cells in log phase of 30 serotypes' strains without 30,31,33 serotypes were used as antigens to vaccinate health New Zealand white rabbits as the standard immunization program,respectively.When the antibody titer reaches higher than 25,the blood would be collected to separate serum antibody,and then remove their cross reactivity among different serotypes by cross absorption test.Using plate agglutination assay to test antibody titers,the results show that all prepared serum sample showed an agglutination titer higher than 1:32,while the agglutination titers of cross reaction were less than 1:4,indicating that these serums can be used as the standard antibody for the detection of S.suis serotype.In September 2016,a virulent bacterial strain was isolated from several pigs with serious meningoencephalitis symptoms in Shandong province,and suspected as S.suis'infection.PCR test of gdh(glutamate dehydrogenase gene)primers as described previously identified this isolate as S.suis,and serological test and PCR typing analysis further confirmed as S.suis serotype 9.Using a mouse model to reproduce its infection characteristics,the inoculated mice showed typical meningoencephalitis lesions,including obvious brain edema,cerebral hemorrhage and covering colloidal exudant.Especially,the half lethal dose of this strain was 1.89 × 102 CFU,which confirmed that this strain was a supervirulent S.suis isolate.Otherwise,drug sensitivity test showed strong sensibilities to vancomycin and cephalosporin antibiotics for this strain.In summary,we prepared 35 standard antibody against 35 serotypes of S.suis,respectively,and isolated a virulent strain of S.suis serotype 9.